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Therapy Project

Thanks Vic. Pages 12 and 13 refer to statically determinate and indeterminate systems. I would argue that the flap reactions at the spar are indeterminate, since both the spar and multiple ribs contribute to resisting drag and lift forces and their associated moments at each attach point. So have fun with the elasticity eqns! The analysis looks ugly to me - - - :cry:
 
Thanks Vic. Pages 12 and 13 refer to statically determinate and indeterminate systems. I would argue that the flap reactions at the spar are indeterminate, since both the spar and multiple ribs contribute to resisting drag and lift forces and their associated moments at each attach point. So have fun with the elasticity eqns! The analysis looks ugly to me - - - :cry:

Me too. LOL. It will give me something to try doing during down time in my 9 hour docket tomorrow.... Beats playing Candy Crush, which I've seen many do.
 
I haven't posted anything for a while mainly because it would be repetitive. I've been building ribs--one at a time--letting them cure overnight, etc. In the time available I've only been able to build 3 or 4 a week. But now I have 4 left to go.

In the mean time, I've been contemplating flaps. The Wag Aero plans show an undetailed "optional" flap 62" long and a chord of 13.25 inches. That makes the chord around 20% of the wing chord.

Chapter 8 of my copy of Abbott and Doenhoff talks about high-lift devices. Flaps of 30% of wing chord or more seem typical. Then I saw the discussion on a fixed vane double slotted flap and those lift coefficient numbers of 3.0 or more. So back to kicking around the idea of a fixed vane double slot:


This template's chord is 31% of wing chord. I'm thinking the flaps could be 7.5 or 8 feet long instead of the 5 feet of the drawings. There is room to move the ailerons out. My concern has always been stress on the rear spar. I've been slogging through stress calculations--I'm very rusty. I'm almost ready to try some empirical destructive testing for fun and profit.

As I researched things related to this I ran across compression struts for a Pitts biplane. I like that approach: plywood web over spruce capstrips snuggly wrapping around the spar. Maybe those could line up with flap hinges.

For a cove I'm already contemplating carbon fiber laid up over a mold. I've done a lot of fiberglass stuff with boats, so that idea seems not so foreign.

Still kicking things around. I may end up with single slot flaps to start, but design the cove to accommodate the double slotted version later, for comparison.[SUB][/SUB]
 
For what it's worth I went with 8' flaps to 50* and 8' with 8 degree droop ailerons that can be pinned up with normal aileron deflection without droop.
 
The built up compression structure is a sound idea. I expect they might need to be next to the control surface fasteners utilizing an angle bracket, but I have not been in a Pitts wing in decades so I should review them.

Flaps, if going slotted I would increase cord as much as possible.
This could allow your double slot flaps to be sized up close to what I am building but with a simpler structure compared to mine.

I would increase the aileron cord to probably 30% and shorten their span to suit. The further they get moved outboard allows their span to be reduced as well. Granted consideration needs to be paid to effectiveness with the reduced speed related to the flaps that will now offer substantial lift.
 
I have found of all the planes which I've flown, the ones which had the longest flaps and wings had the best low speed extra lift capabilities and characteristics. The ones with the shorter flaps, while they did produce lift were more effective in just drag production. Most STOL type planes have long single slotted flaps. As the deflection increases there is more need to pay attention to a leading edge slot for the flap. When the deflection increases further a second slot as you are suggesting enters the picture. My Cub is using a long 110" single slotted flap using the stock PA-18 cross section. The deflection of 56 degrees is showing with tuft testing to be satisfactory, with a smooth air flowing from the trailing edge. With this experience in mind, I would only consider your extra slot if you desire to exceed 56 degrees of deflection. One not STOL airplane which I've flown extensively (DC-9-30) had a double slotted long flap with a leading edge slat. The slow speed capabilities and advantages of the various available settings were very obvious and worthy of your consideration for your plane. The DC-9-10 had a shorter fuselage and no leading edge slat. It seemed to feel better during landings than the -30 with the LE slat.

When increasing the lift using flaps, the center of lift of the entire wing moves aft. When increasing the flap lift further it becomes necessary to consider the authority of the horizontal tail to control the pitch moments. When you also incorporate leading edge slats which move the center of lift forward balancing the extra flap, less consideration to changing tail size is necessary.

When in High School I made a smoke wind tunnel as a science fair project using multiple nozzles for smoke stream distribution. Initially used cigarette smoke which rapidly clogged the nozzles (imagine what it does in your lungs!). When using dry ice it worked beautifully. The air flow was generated with a vacuum cleaner drawing the air through an approximately 2" wide tunnel. I still have this tunnel in the attic.
Perhaps you ought to consider a side therapy project of a smoke tunnel? I would like to visually see just what the airflow paths would be at different deflection angles both with a single slot and a double before I went through with building the full sized parts.

Here are some examples.
https://www.instructables.com/id/How-to-make-a-wind-tunnel/
https://www.grc.nasa.gov/www/k-12/WindTunnel/build.html
https://www.youtube.com/watch?v=9s1b50cxSK0
https://www.youtube.com/watch?v=NIHNtjFJook

If you wish to exceed 56 degrees of flap deflection you will need to address the geometry of the entire actuating system starting with the flap handle all the way to the flaps. The standard Cub system just will not go any further than 56 degrees. At least mine would not without a major alteration.
 
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The idea of a smoke tunnel is an interesting idea and one I should consider for my far from simple approach.
 
Perhaps you ought to consider a side therapy project of a smoke tunnel? I would like to visually see just what the airflow paths would be at different deflection angles both with a single slot and a double before I went through with building the full sized parts.

So much to learn! So much fun to have, and so little time!

Sky, I like the idea....
 
The built up compression structure is a sound idea. I expect they might need to be next to the control surface fasteners utilizing an angle bracket, but I have not been in a Pitts wing in decades so I should review them.

Flaps, if going slotted I would increase cord as much as possible.
This could allow your double slot flaps to be sized up close to what I am building but with a simpler structure compared to mine.

I would increase the aileron cord to probably 30% and shorten their span to suit. The further they get moved outboard allows their span to be reduced as well. Granted consideration needs to be paid to effectiveness with the reduced speed related to the flaps that will now offer substantial lift.

Charlie, here is a photo I saw of the compression rib:
Pitts comp rib.jpg

It came from this blog:

https://mars58superstinker.wordpress.com/tag/pitts-special/page/8/

Vic
 

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A little barnyard engineering follows. It may be my concerns about wing twist are a little overblown.

Basically, through various approaches I come up with calculated loads leading to a maximum of some 1200 pound-inches of torque on the rear spar over a 9 foot long 18 inch chord flap. If that is spread over 4 hinges, then there is 300 pound-inches for each hinge (figuring center of pressure at 1/2 chord).

So I was wondering what my frail-looking ribs could handle. I decided to put a piece of 3/4 ply in a vise and stick the rear spar opening of a partially completed rib, and then apply force at the front spar opening to see what would break.

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At 9 pounds 15 ounces, my plywood-in-a vice failed.

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I reattached the plywood and got up to 10 pounds before the rib started to twist (in a wing, twist is prevented by cross taping).

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So, one rib with only half of the gussets on it can hold a torque of 310 pound-inches on the spar without failing. (31 inch center X 10 pounds).

Then there is the fact that at 14" spacing for each rib, 9 feet of spar is supported by 7.7 ribs. 300 pound-inches times 7.7 ribs is 2100 inch-pounds of demonstrated torque support.

I suppose that is why people put flaps on the 2+2 without giving it much thought--there seems to be quite a bit of reserve just in relying on ribs, not including stiffness of the spar and other reinforcing.

I still want to add a stiffening compression rib near hinge points, belt and suspenders I suppose.

Because I'm a hopeless nerd, the other thing I want to do is empirically verify the torque forces of such a flap at various speeds. I'm contemplating a set up in the back of my pickup that would allow me to measure actual forces at various highway speeds and angles.
 

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A little barnyard engineering follows. It may be my concerns about wing twist are a little overblown.

Basically, through various approaches I come up with calculated loads leading to a maximum of some 1200 pound-inches of torque on the rear spar over a 9 foot long 18 inch chord flap. If that is spread over 4 hinges, then there is 300 pound-inches for each hinge (figuring center of pressure at 1/2 chord).
Remember that in order for the flap to exert twist loads on the spar it has to have two points of attachment. One at the top and the other at the bottom. The actuator push rod is the one at the top and the hinges (however many there are) are the one at the bottom. The bottom force will be distributed among the hinges. The twist force will be concentrated at the push rod. In order to reduce the spar twist load you will need to add actuators spread out along the spar. My 110" flap has two actuators tied together with a cable. For spar twisting protection, place the actuator(s) adjacent to a compression rib which will transfer the twist loads to the front spar where it converts to a low vertical load reduced by the lever arm length of the compression rib.
 
I also will be using two actuators on my 120" flaps. My three hinges hang a stupid long distance below the wing and have structure that travels half way to the front spar. These obviously need additional structure to carry the loads such as you are considering the Pitts style Compression rib. Alternately you can get by with a weldment at each hinge point.
 
Remember that in order for the flap to exert twist loads on the spar it has to have two points of attachment. One at the top and the other at the bottom.

You are right, Sky. For simplicity I was putting imaginary actuators at each hinge. The actuator loads will be different if fewer.

I'm starting to see why Javron put in that reinforcing web aft of the spar.

In any event, I'm contemplating a torque tube to control actuator arms. It seems that would be easy to keep aligned.
 
I am using a torque tube on mine as well. They can be expensive to make light but they do a very good job keeping the action synchronized with the fewest parts.
 
In any event, I'm contemplating a torque tube to control actuator arms. It seems that would be easy to keep aligned.
Good idea, use roller or needle bearings at each support block to reduce actuating loads on the entire system.
The Citabrias use a torque tube mounted in the fuselage with the control arm mounted just outside the fuselage. This then has just one push rod connected to the inboard end of the flap. The flap then is designed to absorb the torsion loads within itself. The Lake LA-4 with it's long flaps is the same.
 
Things are getting tight in my little shop

Having gotten the ribs completed (sanding and finishing, plus cove reinforcement remains), I felt like laying out some spars and marking locations. I see I'm going to need to do some rearranging of things if I want to work efficiently.

20191012_1633331600902429.jpg

I don't have any other shelter for the boat right now, but that would be the obvious solution. Other possibility is to hang the fuselage from the ceiling toward the back during the wing phase.

One thing I'm still looking for is some data on torque tubes, e.g. twisting strength tolerances relative to diameter and tube thickness. For another barnyard experiment I took some scrap 3/4 x .035, stuck one end in a pipe vice, and twisted the other end with a spring-bar torque wrench and a pipe chain fitting. At about 80 lb-ft the end crumpled but there was no twist over 2 feet. Welding actual arms on it would be a better test. I'm wondering if there is some easy-to-find chart that would give recommended dimensions.
 

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How long a torque tube, just across the fuselage driving the inboard of a flap? Or out to the outer or middle flap hinge as well?
The length is quite important in this case.
 
How long a torque tube, just across the fuselage driving the inboard of a flap? Or out to the outer or middle flap hinge as well?
The length is quite important in this case.

I'm trying to get educated enough to weigh the options. Assuming in the neighborhood of 12 feet from approx. midpoint of flap on one wing to the other. That's not a fixed idea, but a starting point.

That's 3.25 pounds approx. of 3/4 4130 at .035. It seems like 1 inch, even with thinner wall, would be more robust. I'm wondering about other materials too.

I think I'll have a handle on the actual flap actuator loads in a few weeks.

Vic
 
Lift Strut Attach hurdles

Before I assembled the wing, I wanted to get some hardware in order to locate it on the spars. I looked at the Lift Strut attachment fittings and decided I ought to just buy them because of the compound curve in the bend.

20191019_1405401640923585.jpg

Great plan, except: "This item is no longer available" at Wag Aero.

So I launched into an experiment at trying to form these pieces with what I had on hand. It's .063 4130 sheet.

First I tried the old vise bending approach:

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It looked sort of OK. But I was nervous about the radius of the bend being too tight. I had to free-hand the compound bend, and it looks sort of ragged.

So I decided to mill a template out of some 1/4 angle I had.

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I put an approximately .14 radius in the template, and then decided to try using heat to see if I could bend the sides more easily.

Of course, I could, but I ended up with nice hammer marks on the upturned sides and tool marks on the back where the work slipped a little.

toolmarks.jpg

So now I have an assortment of tries, and my head is getting sore from the scratching.

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These are critical parts, and sort of rare because I'm building a wood wing. I was looking on ebay to see if I could find something. There are wood spar J-3 fittings, "used", and looking more dinged up than what I have already.

I'm wondering if I need to buck up and fabricate a decent set of dies and get a bigger press. Hammer blows on this sort of thing kind of worry me.
 

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Maybe take 'em to a fab shop that has a press brake with the correct radius dies? I know of one local here that knows aircraft fabrication very well (super-12.com), but no doubt you have options there as well.
 
Maybe take 'em to a fab shop that has a press brake with the correct radius dies? I know of one local here that knows aircraft fabrication very well (super-12.com), but no doubt you have options there as well.

Thanks for jogging my memory. It caused me to remember that I met a guy who is a pilot for an aeronautical machine shop that subs work for Boeing and others. He told me if I needed something made up, the local shop has 5 different presses.

That's exactly the solution.
 
Did you try a stiff backing plate on the back side with thru bolts going through the pattern, piece to be formed, and then the backing plate to form a sandwich ?
 
Did you try a stiff backing plate on the back side with thru bolts going through the pattern, piece to be formed, and then the backing plate to form a sandwich ?

That sounds interesting. I haven't tried that. Maybe I'll give it a try this long weekend.
 
I feel your pain....I bent and cracked a front end loader mounting plate on my tractor and I'm just about ready to go buy a new one since it's 1/2" steel and there are 6 holes I need to align and get back where they are supposed to be so I can bolt it back in place. I can weld it just fine but each time I do the heat changes placement of the bolt holes...I feel like the old Johnny Cash song about trying to get the transmission to bolt up to the engine.

Good luck on yours RV!

Best wishes to the fellow vets on 11/11/2019.
 
Did you try a stiff backing plate on the back side with thru bolts going through the pattern, piece to be formed, and then the backing plate to form a sandwich ?
In addition use a dolly against the part to be bent and hit the dolly with the hammer. This will spread out the impact points reducing the possibility of hammer marks. Make a curved dolly of the proper radius for the compound section.
 
Forming .063 4130 like this is a challenge. Your tooling has to be able to withstand the forming process.
Piper made these out of mild steel. Big difference in the effort to make those curves. There is still a lot of wood wing J3 parts still around.
 
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